1. Field of the Invention
The present invention relates to an optical information recording medium that records and reproduces information at high speed and high density using optical means such as a laser beam, and a method for manufacturing the medium.
2. Description of the Prior Art
Techniques for reproducing or recording high-density information using a laser beam are known, and have been commercialized mainly as optical disks.
Optical disks are roughly divided into read only types, write once types and rewritable types. The read only types have been commercialized as compact disks or laser disks, while the write once types and the rewritable types are commercialized as recording media for document files or data files, for example. The rewritable type optical disks include magneto-optical types and phase change types. The phase change optical disk utilizes the phenomenon that a recording layer changes its phase reversibly between an amorphous phase and a crystalline phase (or between one crystalline phase and another crystalline phase) upon application of a laser beam. Namely, when a laser beam is applied, at least one of a refractive index and an attenuation coefficient of a thin film are changed for recording, and the amplitude of transmitted light or reflected light is changed. As a result, a signal is reproduced by detecting the change in amount of the transmitted light or the reflected light that reaches a detection system.
Note that recently a single sided double layer structure has been proposed with a view to increasing record capacity of the optical disk (see Japanese unexamined patent publication No. 2000-036130, for example).
In addition, research and development has been performed which was aimed at commercializing a double layer optical information recording medium that records and reproduces using a blue color laser. According to this technique, a spot size of a laser beam can be reduced by using a laser beam having a wavelength that is shorter than conventional and an objective lens having a numerical aperture (NA) that is larger than conventional, so that information can be recorded at a higher density.
One of the requirements for a single sided multilayer recording medium is that an information layer that is disposed at the incident side of the laser beam, which is used for recording and reproducing information, has as high transmittance as possible. For example, in the case of a single sided double layer recording medium, when a laser beam passes through the information layer disposed at the incident side and reaches the information layer disposed further inside the medium (i.e., the deeper side), intensity of the laser beam may be reduced. Thus, the laser beam with low intensity will be used for recording on and reproducing from the information layer at the deeper side. Therefore, in order to secure sufficient intensity of the laser beam for recording information on the information layer at the deeper side, the information layer and the like on the front side are required to have especially high transmittance. In order to realize such high transmittance for the information layer that includes at least a recording layer and a reflection layer in this order (when viewed from the incident side of the laser beam), a technique has been studied in which a transmittance adjustment layer made of a dielectric is arranged to contact the side of the reflective layer that is opposite to the incident side of the laser beam. In addition, another technique has also been studied in which refractive indexes and attenuation coefficients of the transmittance adjustment layer and the reflection layer are optimized to realize high transmittance.
Note also that it has been proposed to use a material made of ZrO2—SiO2—Cr2O3 for a dielectric layer that is in contact with the recording layer. For a disk that is required to have a high transmittance, it is essential to make the laser recording layer, which absorbs light and causes reduction in transmittance, as thin as possible. But thinning of the recording layer may cause another problem in that crystallization ability of the recording layer is reduced. As a countermeasure to this problem, a material made of ZrO2—SiO2—Cr2O3 may be used for the dielectric layer that contacts with the recording layer so that the reduction in crystallization ability of the recording layer can be suppressed.
In addition, many magneto-optical recording media and DVD-RAMs are manufactured in large quantities by using a sheet-fed type sputtering device as a mass production device. In this sheet-fed type sputtering device as shown in FIG. 5, a disk substrate 10 is charged into a vacuum chamber (a main chamber 19) via a load lock chamber 11 and transferred to a film forming chamber to form a first layer (a film forming chamber 12 in this case). The disk substrate 10 is processed in the film forming chamber 12 to form a film, and then transferred to another film forming chamber for the forming of a second layer (a film forming chamber 13 in this case). Here, the film forming process is repeated so that desired layers are formed by the disk substrate 10 being transferred to each film forming chamber. After that, the disk 20 with films formed thereon is taken out via the load lock chamber 11 again. Disk substrates 10 are charged successively via the load lock chamber 11 for mass production.
The inventors of the present invention first worked toward development of a double layer optical information recording medium in order to study a single sided multilayer recording medium. As a material for the dielectric layer, the above-mentioned ZrO2—SiO2—Cr2O3 was used because it accelerates crystallization, even if the recording layer was made of a thin film.
However, in a mass production study using the above-mentioned sheet-fed type sputtering device having this structure, a problem was found in that peeling of a film can occur easily after a humidity-proof test. As a result of detailed study, it was found that film peeling can occur easily at the interface between the dielectric layer at the recording layer side and the recording layer, at the interface between the dielectric layer at the incident side of the laser beam and the recording layer, and at the interface between the dielectric layer at the reflection layer side and the reflection layer. In addition, it was found that film peeling can occur easiest at the interface between the dielectric layer at the recording layer side and the recording layer. This film peeling can be suppressed by increasing the amount of Cr2O3 in the material that forms in the dielectric layer. However, the increase in the amount of Cr2O3 may cause a reduction in transmittance, having a large influence on recording and reproducing characteristics of the information layer that is disposed at the deeper side (when viewed from the incident side of the laser beam). For this reason, it has been found that the material composition of the dielectric layer cannot be changed easily.
In addition, the film forming tact affects the cost of mass production. Therefore, when manufacturing recording media in large quantities using the sheet-fed type sputtering device, it is difficult to perform vacuum discharge with respect to a substrate supplied from the load lock chamber and the load lock chamber itself for a sufficient period of time (vacuum time may be approximately 2–3 seconds, for example). Therefore, moisture from outside of the chamber or that is attached to the substrate can easily enter the film forming chamber. The moisture may deteriorate the corrosion resistance of optical disks to be produced. It has also been found that an optical disk produced by the above-mentioned conventional method has corrosion resistance that is inferior to an optical disk produced by batch type mass production equipment.
As noted above, to realize both the suppression of film peeling and improvement of transmittance, a layer structure of a disk must be optimized as well as the material composition of the dielectric layer.